"Mixed Ion-Electron Conducting Polymer Architectures for Energy Storage Applications"

Pratyusha Das,¹ Alexandra Zele,² Phong H. Nguyen,³ Michael L. Chabinyc,^1,2 Rachel A. Segalman^1,2,3*

¹Materials Research Laboratory, University of California, Santa Barbara, California 93106
²Materials Department, University of California, Santa Barbara, California 93106
³Department of Chemical Engineering, University of California, Santa Barbara, California 93106

Mixed ion-electron conducting polymers have garnered significant attention in recent years for applications in batteries, electrochemical transistors and many such electrochemical devices. Contradictory design rules for electron and ion conduction create synthetic challenge, making a small library of such mixed conducting polymers. While conjugated polymers with Li⁺-ion conducting oligoether side chains have previously shown mixed conducting ability, their ionic conductivity (∼10^-7 S/cm) lags far behind their electronic counterpart (∼10^-1 S/cm). In this regard, our group has recently shown that cationic conjugated polyelectrolytes such as polythiophenes with alkyl side chains attached to imidazolium pendant units and bulky TFSI^- counterions can lead to long-range polymer ordering and diffuse ion interactions resulting in high Li⁺ ionic conductivity (∼10^–3 S/cm at 80 °C and t_Li+ ~ 0.32) and electronic conductivity (~ 10^-4 S/cm). In this work, we have further engineered the polymer architecture to incorporate alkoxy side chains with imidazolium pendants to increase stability in the doped state, thereby further enhancing electronic conductivity and electrochemical stability. We observed that with Br^- counterions, room temperature electronic conductivity was increased by several orders of magnitude, up to 10^-1 S/cm upon vapor doping with HTFSI, while still retaining good ionic conductivity (~ 10^-3 S/cm at 80°C). Additionally, electrostatic complexation with insulating Na⁺PSS^- resulted in a coacervate formation where properties of CPE aggregation was retained, as observed by UV-Vis spectroscopy. We demonstrated considerable enhancement of mixed ion-electron conduction on incorporation of alkoxy side chains compared to alkyl side chains. Our current efforts are focused on measuring Li⁺ ion diffusion using PFG NMR, morphological investigation using GIWAXS and finally application as complex battery binders upon electrostatic complexation with an oppositely charged polymeric ionic liquid (PIL), which has also been previously established by our group.

References:

(1)        Pace, G.; Nordness, O.; Asham, K.; Clément, R. J.; Segalman, R. A. Impact of Side Chain Chemistry on Lithium Transport in Mixed Ion–Electron-Conducting Polymers. Chem. Mater. 2022, 34 (10), 4672–4681. https://doi.org/10.1021/acs.chemmater.2c00592.

(2)       Pace, G.; Nordness, O.; Nguyen, P. H.; Choi, Y.-J.; Tran, C.; Clément, R. J.; Segalman, R. A. Tuning Transport via Interaction Strength in Cationic Conjugated Polyelectrolytes. Macromolecules 2023, 56 (15), 6078–6085. https://doi.org/10.1021/acs.macromol.3c01206.

(3)       Le, M. L.; Warner, C.; Segalman, R. A.; Chabinyc, M. L. Role of Complexation Strength on the Photophysical and Transport Properties of Semiconducting Charged Polymer Complexes. Chem. Mater. 2023, 35 (11), 4449–4460. https://doi.org/10.1021/acs.chemmater.3c00627.

(4)       Pace, G.; Zele, A.; Nguyen, P.; Clément, R. J.; Segalman, R. A. Mixed Ion–Electron-Conducting Polymer Complexes as High-Rate Battery Binders. Chem. Mater. 2023, 35 (19), 8101–8111. https://doi.org/10.1021/acs.chemmater.3c01587.